The present invention relates to a tuning circuit for a high frequency receiver having a superheterodyne oscillator whose frequency can be varied by means of a tuning voltage in order to tune the receiver to a desired station.
In such circuits, the tuning voltage, which is a direct voltage, is generated by a comparison circuit which effects a comparison between two numbers. The first number identifies the counter state of a counting device which periodically counts the oscillations of the superheterodyne oscillator on the basis of the intermediate frequency. This number thus constitutes the received frequency, or possibly the received channel or station. The second number is fed to the comparison circuit via an input keyboard provided with a series-connected coder and identifies the desired channel or station to which the receiver is to be tuned. The comparison circuit has three outputs, one output for the comparison result "equality" and one each for the two non-equality results "greater" and "less".
In the circuit disclosed in U.S. application Ser. No. 708,754, filed by the present applicant and Dieter Rottmann and Stephan Wuttke on July 26, 1976, the comparison is effected by a multiplex operation and the comparison circuit is composed of only a single comparator which can effect a comparison with respect to only one digit, or level of significance, of the numbers at a time. This comparator receives, in succession, the representations of the digits of the two numbers to be compared, beginning with the most significant digit.
If in the described tuning circuit the two numbers are unequal, the tuning circuit, and thus the frequency of the superheterodyne oscillator, is changed until the frequency identified by the number fed in has been attained. The rate at which the tuning voltage changes is relatively great if the two numbers to be compared differ from one another in all digit positions. The rate of change becomes less as more digits of the two numbers, beginning with the most significant digit, become equal. When all digits are equal, the frequency will no longer be changed. The tuning rate thus decreases as more digits become identical, this result being a function of the evaluation circuit provided in the above-cited application. The successive rates there are proportional to 1/2 . . . 1/n, where n = number of digit positions, e.g. 5. It has been found that this tuning process takes a relatively long time, which many operators consider to be a drawback.